Digital Calibration Method and Apparatus for A/D or D/A Converters
The present invention relates generally to calibration methods and apparatus for calibrating A/D (Analog-to-Digital) or D/A (Digital-to-Analog) converters, and more particularly to an improved calibration method and apparatus for performing digital calibration on A/D and D/A converters employed in IC testers which test various electrical characteristics of IC (Integrated Circuit) devices.
In order to ship factory-fabricated IC devices as final products with performance and quality assurances, it is generally necessary to extract some or all of the IC devices and examine predetermined electrical characteristics of the extracted IC devices at manufacturing and testing stages in the factory. Normally, the electrical characteristics of the IC devices are examined by the so-called IC testers. Typically, the IC tester first sends predetermined test pattern data to each of the IC devices to be tested (hereinafter xe2x80x9cto-be-tested IC devicexe2x80x9d) and then reads output data produced from the IC device in response to the test pattern data sent thereto, on the basis of which the tester examines the IC device""s electrical characteristics and determines whether or not the to-be-tested IC device has any trouble with its essential behavior and functions.
The tests performed by the IC testers can be roughly classified into a direct current test (i.e., DC measurement test) and a function test. In the direct current test, each to-be-tested IC device is checked for presence of trouble with its essential behavior by a DC measurement section applying a predetermined voltage or current to input/output terminals of the IC device. In the function test, on the other hand, predetermined test pattern data is fed from a pattern generation section to the input terminal of the to-be-tested IC device and output data produced from the IC device in response to the test pattern data is analyzed to determine whether the to-be-tested IC device has any trouble with its essential behavior and functions. More specifically, the function test supplies the to-be-tested IC device with various input signals, such as an address, data, write-enable signal and chip-select signal, while variously changing input conditions of the signals and then inspects timing, amplitudes, etc. with which the IC device generates output signals in response to the input signals. Because many A/D and D/A converters are generally employed in each of such IC testers, accurate adjustment or calibration (including gain calibration and offset calibration) of these A/D and D/A converters is extremely essential for high-accuracy tests of the factory-fabricated IC devices.
So far, the adjustment or calibration of the AD and D/A converters has been performed either in a manual manner based on manipulation of a trimmer resistor by a human operator or in an analog manner using an auxiliary D/A converter and digital potentiometer.
However, the above-mentioned manual calibration scheme using the trimmer resistor is disadvantageous in that it would take considerable amounts of time and human labor and thus can not be used suitably for the IC testers requiring calibration of many A/D and D/A converters. The analog calibration scheme, on the other hand, often proves very efficient because it can be performed automatically without any human labor. But, the accuracy of the analog calibration tends to deteriorate due to the nature of analog components or elements used. Further, because the analog calibration scheme calculates calibration coefficients through the xe2x80x9cbinary searchxe2x80x9d or xe2x80x9cleast squarexe2x80x9d method, it would take an extra amount of time for the calculation of the calibration coefficent.
It is therefore an object of the present invention to provide a digital calibration method and apparatus which can perform calibration of A/D and/or D/A converters in a digital manner. More specifically, the object of the present invention is to provide a digital calibration method and apparatus which can perform digital calibration of a great number of A/D and/or D/A converters at increased efficiency.
To accomplish the above-mentioned object, the present invention provides a digital calibration method for an analog-to-digital converter, which comprises the steps of: inputting at least two different reference analog signals to an analog-to-digital converter to be adjusted or calibrated; calculating calibration parameters on the basis of digital converted signals outputted by the analog-to-digital converter in response to input of the corresponding reference analog signals thereto; and performing calibrating arithmetic operations on an output digital signal from the analog-to-digital converter using the calibration parameters calculated by the calculating step. In the present invention, the above-mentioned calibration parameters may include an offset calibration value and gain calibration coefficient.
According to the present invention, predetermined calibration parameters, such as an offset calibration value and gain calibration coefficient, are calculated on the basis of digital converted signals outputted by the analog-to-digital converter in response to input of the corresponding reference analog signals, and then an output digital signal from the analog-to-digital converter is digitally adjusted or calibrated on the basis of the thus-calculated calibration parameters. With such arrangements, the present invention can eliminate a need for analog components or elements and thus can avoid unwanted influences, such as measurement errors, that would be caused by such analog elements. Further, because the offset calibration value and gain calibration coefficient are determined on the basis of the digital converted signals corresponding to the two reference signals, the calibration can be performed with increased accuracy at increased speed as compared to the conventional binary search or least square method. Therefore, the present invention can significantly reduce the necessary calibrating time even in a case where the calibration is performed on a great number of A/D converters employed in an IC tester or the like.
The present invention also provides a digital calibration method for a digital-to-analog converter, which comprises the steps of: inputting at least two different reference digital signals to a digital-to-analog converter to be calibrated; calculating calibration parameters on the basis of analog converted signals outputted by the digital-to-analog converter in response to input of the corresponding reference digital signals thereto; and performing calibrating arithmetic operations on the basis of an input digital signal to the digital-to-analog converter using the calibration parameters calculated by the calculating step. The above-mentioned calibration parameters may include an offset calibration value and gain calibration coefficient. Further, the aforementioned step of calculating may include a step of converting, into a digital signal, each of the analog converted signals outputted by the digital-to-analog converter in response to input of the corresponding reference digital signal and a step of calculating the calibration parameters on the basis of each of the digital signals provided by the step of converting.
According to this method, predetermined calibration parameters, such as an offset calibration value and gain calibration coefficient, are calculated on the basis of analog converted signals outputted by the digital-to-analog converter in response to input of the corresponding reference digital signals, and then an input digital signal to the digital-to-analog converter is digitally adjusted or calibrated on the basis of the thus-calculated calibration parameters. With such arrangements, the inventive method can eliminate a need for analog elements and thus can avoid unwanted influences, such as measurement errors, that would be caused by such analog elements. Further, the inventive method can significantly reduce the necessary calibrating time even in a case where the calibration is performed on a great number of D/A converters employed in an IC tester or the like.
The present invention may be implemented not only as a method invention as set forth above but also as an apparatus invention. The present invention may also be practiced as a program for execution by a processor such as a CPU or DSP, as well as a storage medium storing such a program.